In this study, the ozone (O-3) formation in China's northwest city of Xi'an and surrounding areas is investigated using the Weather Research and Forecasting atmospheric chemistry (WRF-Chem) model during the period from 22 to 24 August 2013, corresponding to a heavy air pollution episode with high concentrations of O-3 and PM2.5. The model generally performs well compared to measurements in simulating the surface temperature, relative humidity, and wind speed and direction, near-surface O-3 and PM2.5 mass concentrations, and aerosol constituents. High aerosol concentrations in Xi'an and surrounding areas significantly decrease the photolysis frequencies and can reduce O-3 concentrations by more than 50 mu g m(-3) (around 25 ppb) on average. Sensitivity studies show that the O-3 production regime in Xi'an and surrounding areas is complicated, varying from NOx to VOC (volatile organic compound)-sensitive chemistry. The industrial emissions contribute the most to the O-3 concentrations compared to biogenic and other anthropogenic sources, but neither individual anthropogenic emission nor biogenic emission plays a dominant role in the O-3 formation. Under high O-3 and PM2.5 concentrations, a 50 % reduction in all the anthropogenic emissions only decreases near-surface O-3 concentrations by about 14 % during daytime. The complicated O-3 production regime and high aerosol levels pose a challenge for O-3 control strategies in Xi'an and surrounding areas. Further investigation regarding O-3 control strategies will need to be performed, taking into consideration the rapid changes in anthropogenic emissions that are not reflected in the current emission inventories and the uncertainties in the meteorological field simulations.